With the development of portable electronic devices such as cell phones and notebook personal computers and the commercialization of electric vehicles in recent years, demand is increasing for small, lightweight and high capacity secondary batteries and capacitors. Currently, high capacity secondary batteries and capacitors that can fulfill the demand commonly employ LiCoO2, LiNiO2, LiMn2O4 and the like as positive electrode active materials.
However, these positive electrode active materials have the following drawbacks. LiCoO2 has low thermal stability in the charged state. LiNiO2 has a capacity higher than that of LiCoO2, but is less thermally stable than LiCoO2 in the charged state. Furthermore, LiMn2O4 has high thermal stability in the charged state, but has a capacity per volume smaller than that of LiCoO2.
Under the circumstances, in order to achieve both thermal stability of LiMn2O4 and the high capacity of LiNiO2, lithium-containing composite oxides have been proposed that has the layered crystal structure of LiNiO2 and in which a certain amount of Ni has been substituted by Mn having high thermal stability (for example, Patent Documents 1 to 3).
In particular, Patent Document 3 discloses a method for producing a lithium-containing composite oxide as described above that includes a process in which raw material compounds are mixed and baked, and thereafter the mixture is washed with water and dried. According to Patent Document 3, the method removes impurities and by-products attached to the baked product obtained by baking the mixture of raw material compounds, thereby providing a lithium-containing composite oxide having excellent thermal stability.